1. Field of the Invention
The present invention relates generally to intraocular lenses, and more specifically to intraocular lenses for providing accommodative vision to a human or animal subject.
2. Description of the Related Art
A human eye can suffer diseases that impair a person's vision. For instance, a cataract may increase the opacity of the lens, causing impaired vision or blindness. To restore the patient's vision, the diseased lens may be surgically removed and replaced with an artificial lens, known as an intraocular lens (IOL). An IOL may also be used for presbyopic lens exchange.
The simplest IOLs have a single fixed focal length, or, equivalently, a single fixed power. Unlike the eye's natural lens, which can adjust its focal length and/or axial location within a particular range in a process known as accommodation, these single focal length IOLs cannot generally accommodate. As a result, distant objects may appear in focus, while objects at closer distances appear blurred.
An improvement over fixed, single focal length IOLs is an accommodating IOL (AIOL), which can move axially and/or adjust its optical power within a particular range. As a result, the patient can clearly focus on objects in a range of distances away from the eye, rather than at a single distance. This ability to accommodate is of tremendous benefit for the patient, and more closely approximates the patient's natural vision than a single focal length IOL.
When the eye focuses on a relatively distant object, the lens power is at the low end of the accommodation range, which may be referred to as the “distant” or “far” power. When the eye focuses on a relatively close object, the lens power and/or position is at the high end of the accommodation range, which may be referred to as the “near” power. The accommodation range or add power, as used herein, is defined as the actual or effective near power provided by a lens or optic (e.g., the natural lens or the optic of an IOL) minus the far power provided by the lens or optic. In general, an accommodation range of 2 to 4 Diopters is considered sufficient for most patients.
The human eye contains a structure known as the capsular bag, which surrounds the natural lens. The capsular bag is transparent, and serves to hold the lens. In the natural eye, accommodation is initiated by the ciliary muscle and a series of connective fibers known as zonules. The zonules are located in a relatively thick band mostly around the equator of the lens, and impart a largely radial force to the capsular bag that can alter the shape and/or the location of the natural lens and thereby change its actual or effective power.
In a surgery in which the natural lens is removed from the eye, a small opening is typically made in the front of the capsular bag through which lens material is typically broken up and vacuumed out of the eye, the rest of the capsular bag being left intact. The remaining capsular bag may be extremely useful for an accommodating intraocular lens, in that the eye's natural accommodation is initiated at least in part by an ocular force produced by the ciliary muscle, zonules, and/or capsular bag. The capsular bag may be used to house an accommodating IOL, which in turn can change shape or optical power of the IOL, and/or shift or axially move the optic of the IOL in some manner to affect the location of the image plane of the optic.
In general, the IOL includes an optic, which refracts and/or diffracts light that passes through it and forms an image on the retina, and a haptic or support structure, which mechanically couples the optic to the capsular bag. During accommodation, the zonules exert a force on the capsular bag, which in turn exerts a force on the optic. The force may be transmitted from the capsular bag directly to the optic, or from the capsular bag through a haptic to the optic.
A desirable optic for an accommodating IOL is one that changes shape or axially moves in response to an ocular force produced by a squeezing or expanding radial force applied largely to the equator of the optic (e.g., by pushing or pulling on or near the edge of the optic, circumferentially around the optic axis). Under the influence of an ocular force, the optic may bulge slightly in the axial direction, producing more steeply curved anterior and/or posterior faces, and produce an increase in the power of the optic. Likewise, an expanding radial force may produce a decrease in the optic power by flattening the optic. This change in power is accomplished in a manner similar to that of the natural eye.
One challenge in providing an effective AIOL is that of effectively transferring a limited amount of ocular force available from the ciliary muscle or capsular bag of an eye to the optic of the AIOL. Typically, the available ocular force is transferred through a haptic or support structure that absorbs a certain amount of the available energy provided by the ocular force. There is a need to provide haptic or support structures in AIOLs that reduce the amount of energy transferred to that structure so that more of the available force may be converted to changing the shape and/or axial position of the optic portion of the AIOL.